FEMA P-50 – Cascadia Risk by BJ Cure

Home Earthquake Vulnerabilities: Soft Stories

This photo shows a classic soft story failure of a wood-framed residential structure. The building on the left used to look similar to the building on the right.

“Soft stories” are a common cause of catastrophic earthquake damage in many types of structures, including houses. Identifying and addressing a soft-story vulnerability is important for both home and building owners. It is potentially very dangerous and represents a high economic risk as well.

The term “soft story” has a technical background that I won’t go into here. A “weak story” or “open front” building means essentially the same thing. Because a building needs shear walls, or some other type of seismic force resisting system, to bring seismic forces to the ground, a building lacking walls on one or multiple sides of it can be particularly vulnerable during the shaking that accompanies a strong earthquake, as it lacks strength and/or stiffness to adequately resist those forces.

If you’re from California, you may have heard the term, “soft story”. Many people from California are very familiar with the term, in fact. Soft story buildings were a significant source of earthquake damage in both the 1989 Loma Prieta and 1994 Northridge earthquakes (the last two “big” earthquakes in California). 16 people died in the Northridge Meadows apartments in 1994, a building with tuck-under parking at the lowest level representing a severe soft story condition.

Jurisdictions in California have identified and required seismic upgrades to these types of structures, because they’ve represented a significant percentage of lives lost in past earthquakes. Los Angeles, San Francisco, and most recently, Pasadena, have gone this route.

Meanwhile, here we are in the Pacific Northwest, and I don’t hear these building types talked about much. While Portland is focusing on requiring seismic upgrades to URM (unreinforced masonry) buildings, the Northwest also has plenty of other vulnerable building types, including many soft story buildings. Many people live and work in these homes and buildings, and don’t realize the risk that these buildings represent.

What is a soft story?

For wood-framed buildings, a soft story typically means a structure lacks walls on at least one exterior face of the building, at the bottom level. While a soft story can occur at an upper story, it is more common at the first floor and is far more dangerous at the first floor.

Soft stories are common at the first floor level in buildings due to garages, tuck-under parking, and open storefronts in retail spaces.
Soft stories are more dangerous at the lowest level, as this level has to resist all the seismic inertial forces working their way to the ground from the upper levels.

Soft stories come in numerous shapes and sizes. For houses, a soft story often occurs at a garage with a living space above.

A garage with a second floor above represents a common soft story condition with houses.

A living space over a garage, or another soft story condition, doesn’t necessarily mean a house is vulnerable during an earthquake. The following are important considerations:

The soft story condition may have been addressed in the design of the house. Current building codes require some type of seismic force resisting system to address this common condition. These include narrow wood shear walls with holdowns, a wood portal frame system, or less common engineered solutions like a steel moment frame or a “3 sided diaphragm” (essentially designing the 3 strong sides of the garage to resist the forces and the induced rotation).
Soft stories vary in their hazard. Some soft stories are “softer” than others. Even a home with an apparent severe soft story condition on one exterior face may have enough redundancy with interior walls that it isn’t at high risk of collapse in reality.
A soft story at the first floor level gets more dangerous the more stories there are above it.
A soft story is one of many seismic risk factors. The condition gets more dangerous when combined with other seismic vulnerabilities.

Soft stories and the age of a house

Newer homes are less likely to be vulnerable due to a soft story. This is for reasons related to building codes and modern construction, mentioned above. Building codes in general made significant changes in the 1990’s addressing seismic details in wood-framed construction. In the Pacific Northwest, the early 1990’s also represented a “seismic shift”, so to speak, as the Cascadia Subduction Zone and its projected design ground accelerations worked their way into our building code. This means houses newer than the mid-1990’s represent a much lower seismic risk in general, even homes with soft stories. However, this is a general statement, and I sometimes encounter exceptions.

Houses newer than the mid-1990’s should have been built to take a soft story condition into account.

Other soft story conditions

Soft stories exist in numerous other conditions with houses. The appeal of an “open floor plan” has always existed, for example. Many houses built in the 1960’s and 1970’s have an architectural style with an exterior wall line almost completely consisting of windows on one side. FEMA P-50 (a seismic risk assessment methodology for houses that I use) flags two-story houses as higher risk if an exterior wall line at the lowest level consists of less than 25% wall segments. For 3-story houses, that number increases to 40%.

Large old houses with multiple remodels

Another common condition is a large, old house that has been remodeled multiple times. Often because an open floor plan is desirable, many old homes have had numerous interior walls removed. These walls add redundancy and help resist seismic forces, even if they were not designed or intended to do so. Sometimes, exterior windows were added and exterior shear wall strength has been reduced.

Many of these homes have been beautifully remodeled and have seen a great increase in market value, but they’ve ironically created a soft story condition, or something similar, and have increased the home’s seismic risk.

A soft story is sometimes created with an addition to a house, as apparently shown in the photo below.

A soft story condition at the rear of a home in the Portland area. Notice the posts with no bracing or walls for seismic support. This was likely the result of an addition, perhaps built in the 1980’s before seismic risk was taken as seriously by building jurisdictions in the area.

Split Level Houses

Much could be written about split level houses, which I won’t do at this time. Split level houses often attract more seismic damage than the average home, due to the discontinuity of floor and/or roof levels. A split level home combined with a soft story can result in the two-story portion of the house pulling away from the rest of the house and collapsing.

An earthquake engineer can look at this house and see a soft story vulnerability on the left side (at the front of the garage) and a smashed cripple wall on the right side. The two-story portion is leaning and close to collapse. The right side of the house apparently had a weak cripple wall that failed during the earthquake. The cripple wall failure is evident by the roof line a few feet lower than it should be against the two-story portion, and front porch stairs that remain after the main part of the house dropped a few feet.

Semi-Soft Stories

For lack of a better phrase, some soft story conditions come with a moderate, or low, seismic risk, compared to other obviously dangerous soft story conditions. Many old homes fit this criteria: they have a decent amount of exterior wall segments (perhaps around 25% based on the FEMA P-50 guideline previously mentioned), but the old shiplap or 1x plank siding just isn’t as strong or ductile as modern, well-nailed plywood sheathing.

In these situations, I try to communicate to homeowners that the risk is lower, but not nonexistent. Whether to seismically upgrade in these situations is a personal decision based on risk tolerance and economics.

I typically would classify a home similar to that shown in this picture as having a “semi-soft” story. However, you can see that this structure is severely damaged and is close to collapse. My suspicion is that the ground accelerations that caused this damage were severe. I wasn’t there, though- this photo was taken after the South Carolina Earthquake of 1886.

Soft Stories combined with other vulnerabilities

A soft story condition combined with other seismic vulnerabilities is particularly dangerous. This combination can push a house past the brink of collapse. Other structural vulnerabilities like a deteriorating foundation, lack of foundation anchorage, or weak cripple walls could make a house more likely to have catastrophic damage when combined with a soft story. Geological hazards such as soft soil prone to liquefaction and/or lateral spreading, or slope instability, are also dangerous when combined with a soft story condition.

While this photo is a somewhat “textbook” example of a soft story failure at the front of a garage, slope instability contributed significantly to this collapse. If the ground shifts enough during an earthquake, it of course puts extra demand on an already vulnerable structure.

Seismic risk involves many variables

Besides addressing the risk of soft story vulnerabilities with houses, this post should also draw attention to the fact that seismic risk is a complex interaction of many risk factors.

For homeowners or potential buyers concerned about seismic risk, I recommend FEMA P-50 seismic risk assessments because they address the numerous known structural and geological vulnerabilities with any specific house. The methodology is simplified, but it quantifies risk at a relatively low cost and even helps identify how a home would perform after constructing a retrofit that mitigates specific earthquake vulnerabilities, such as a soft story, lack of foundation anchorage, or a weak cripple wall.

For more information about FEMA P-50 seismic assessments, click here.

Near collapse of a weak story structure in the Marina District of San Francisco after the 1989 Loma Prieta earthquake. The Marina District experienced strong, amplified ground accelerations due to soft soil.

How can a soft story be strengthened?

There are many possible ways to add adequate strength and stiffness to a soft story in an existing building. For houses, plywood shear walls are the least expensive solution and often the best. I also often recommend and design steel “moment” columns. Usually, a new reinforced concrete foundation is required to support these new systems. These are the two systems I most commonly work with and will focus on these two.

Other systems such as wood portal frames, steel moment frames, braced frames, and concrete and masonry shear walls could also be used if it made sense to do so.

While strengthening weak cripple walls and adding foundation bolts doesn’t necessarily require engineering, a soft story does.

New plywood shear walls

If there is room on an existing wall segment to add plywood and holdowns to create a shear wall, this is the least expensive approach. A new plywood shear wall with a new footing is often required, however, assuming the intent is to build the new wall to current seismic code standards.

A structural engineer can determine what the minimum or recommended wall length would be, and an appropriate location for the wall can be determined by the engineer and homeowner.

The following three photos show a soft story condition strengthened with a new plywood shear wall and concrete footing.

This Google image shows a house with a soft story condition at the front of the garage. The house was built in the ’50’s and the owner’s child’s bedroom is directly above the garage. The homeowner elected to install a new plywood shear wall and footing at the front of the garage on the left side. This was the least expensive option, even though he had to replace the double garage doors with a single door.

This is the exterior siding on a new plywood shear wall at the front of the garage for the house in the previous picture.

An interior view of the same plywood shear wall. A new reinforced concrete foundation was created by cutting a new trench in the garage floor at the front of the garage. A small concrete stem wall was placed above the foundation which was flush with the top of the existing garage concrete slab.

Steel “moment” columns

Sometimes, particularly in a soft story condition at the front of a garage, there is no room to place a new plywood shear wall, or it’s not desirable to modify the garage door or the space inside the garage. In this case, a steel “moment” column or “cantilevered” column with a new concrete footing is often the best approach.

Think of the new steel column like a vertically oriented, extremely rigid diving board. While columns typically are intended to take vertical loads, a moment column is designed to take seismic loads (and usually no vertical loads at all). A moment column needs a new concrete footing with a large enough mass to resist the overturning or rocking that the cyclical seismic forces place on it.

For buildings in general, steel moment frames are more conventional than moment columns. A moment frame consists of two steel columns and a steel beam. A moment frame can be used when retrofitting a house for a soft story condition, but it is often difficult to fit and a moment column is often simpler.

This house had a severe soft story condition at the front with the tuck-under parking and two stories above. There wasn’t space for a plywood shear wall, so a steel column and large concrete footing was placed on the left side with a new wood beam over the garage to act as a collector for the seismic forces to transfer to the steel column.

Recent developments

A structural engineer in the San Francisco area has developed an “Earthquake Resisting Column” (ERC) with a “structural fuse” at the top of the column. The “fuse” is essentially a carefully designed rocker that dampens seismic forces and allows for design of a much smaller steel column and footing. He designed it primarily for the stereotypical tall and skinny classic San Francisco style house, where sometimes only inches of room exist each side of the garage door for a new steel column.

I’ve designed my first seismic retrofit using this type of column on a house in northeast Portland which will be installed soon. A video of this type of column in testing is shown here.

This home in northeast Portland has minimal walls at the front with a living space above. Many soft story mitigation measures were discussed with the owner, but we landed on an ERC by the Soft Story Brace Company. The new steel column will replace the far right double post shown in this photo and will have a new reinforced concrete footing.

For more information about seismic risk assessments and retrofitting, please see the Cascadia Risk Solutions website.

Home Earthquake Vulnerabilities: Hillside Home Recommendations

Home and building owners, as well as renters, in hillside neighborhoods need accurate information about their earthquake risk so they can make informed decisions.

Hillside homes can have high, even catastrophic, earthquake risk. The previous two posts discussed common geological and structural problems with hillside homes.

But if you live in or own a hillside home, what should you do? Move away? Just live with the risk, and hope “The Really Big One” doesn’t happen in your lifetime? Get the house seismically upgraded? Get more information?

Yes, those are the four options that come to my mind:

1. Move Away?

I’ll address this first, because many reading this are concerned about earthquake risk. Of those of you who own and/or live in a hillside home, I’m guessing a high percentage of you didn’t know how dangerous this type of home can be in an earthquake. It’s likely no one told you anything about earthquakes when you bought or moved into the house, in fact, it may not have even entered your mind at the time. But here you are, and now you are thinking of moving, perhaps.

Moving away makes sense if:

You are confident the house is dangerous, and
You’re confident that it would be too expensive for your budget to fix, and/or
You aren’t too attached to the home, or maybe you don’t even like it.

Moving out of a hillside home that you perceive to be dangerous makes more sense, in my opinion, than staying and living with the risk. But you may want to consider my last point (#4 below) before moving.

2. Keep the house and live with the risk?

I certainly don’t recommend this. But there are some situations that are less risky than others.

If you own multiple homes and are rarely occupying the house, risk (at least life-safety risk) is obviously lessened simply due to the fact that you aren’t around much. If you don’t have kids and travel often, that’s a similar situation.

If you have a family, especially with a spouse or kids staying home during the day, you genuinely could be putting their lives at risk by not addressing potential seismic vulnerabilities in the place where most of their time is spent.

If your house is looming over your neighbor’s house down the hill, they could be at risk also due to your home’s earthquake vulnerabilities. You may both have earthquake insurance policies (unlikely), but that can’t make up for loss of life. Just something else to think about before you decide to do nothing.

Interestingly, there may be an economic argument against doing nothing also. Suppose your hillside home is worth a million dollars. Let’s say the chance of a severe earthquake affecting this house in the next 50 years is 20 percent (this is in the ballpark of what seismologists have estimated). Suppose the odds of collapse of the home during the earthquake are 50 percent (arbitrary number), with severe damage likely even if it does not collapse. A retrofit costing in the tens of thousands of dollars, or even $100,000, isn’t necessarily unreasonable in this circumstance, for those with the available capital and desire to keep the home.

There are situations where a retrofit could be more costly, but this would be difficult to know without more information (which is why I like Option #4 below).

It’s also possible that the house has low risk of seismic damage. In that case, it may be reasonable to live with the risk. But how would you know this? You probably need a specific assessment to be sure.

A collapsed California house after the Northridge Earthquake. This will likely happen to some hillside homes in Oregon and Washington when we get our “Big One”. Please take a look at this picture and ponder if you are okay living with this risk before choosing to do so.

3. Have your home seismically upgraded?

Of course, I recommend a seismic upgrade for many hillside homes. I’m concerned about the risk of a Cascadia Megaquake, and what it will do to hillside neighborhoods. This is why I’m writing this and specializing in this type of work.

But the choice to upgrade the home has to work economically. Hillside home seismic upgrades can be expensive, and not only does the money need to be there to pay for the upgrade, but the benefit should be worth the cost to the homeowner. I recommend spending the time necessary up front so you have a good ballpark figure of the cost.

Don’t Mess Around With Cascadia

I strongly believe in conservatism with seismic upgrades. Our Cascadia Subduction Zone could produce a magnitude 9.0+ earthquake that could last 3 to 5 minutes. This earthquake will last much longer than earthquakes that have caused the collapse of hillside homes in California in recent decades. Homes that have poor seismic force resisting systems (such as stilts with wood bracing) could degrade with each cycle of ground shaking, and there could be hundreds of cycles in this type of earthquake.

My point is this: if you are going to do an upgrade, do something that will actually work. Don’t just pay someone a few hundred bucks to take a quick look at your house and give you a few cheap recommendations. Count the cost ahead of time and be willing to pay for the thorough upgrade that you really want, that will really do what it needs to do when the ground shakes longer than an average pop song.

Hillside home seismic upgrades are complex, and involve much more than just “attaching the home to the foundation”.

You will need a structural engineer who specializes in hillside building seismic upgrades. I’m trying to be that guy because there’s a need there, but if you find someone else who qualifies, that’s great! More engineers need to be doing this, in fact, we really need an “army” of specialty engineers and contractors retrofitting homes and buildings ahead of the earthquake who are passionate about this kind of life-saving work.

You will, in many cases, need a geotechnical engineer also. Geological risks can’t be ignored and can sometimes drive the cost of seismic rehabilitation through the roof. If landslide risk is high, mitigation may be expensive or even virtually impossible. Make sure you figure this out with a geotechnical investigation, and make sure their recommendations are followed in the seismic upgrade. Or, if landslide risk is apparently low, at least have a structural engineer consider slope stability in the seismic upgrade including a conservative design with a new foundation if needed.

A stepped foundation collector I designed for a hillside home **built in 2001**. The intent of this design is to prevent the stepped shear wall failure (described in the previous blog post) by directing seismic loads into the high part of the foundation. I communicated to the homeowner that this type of failure was unlikely for her house, but I couldn’t rule it out. **She wanted to strengthen the house.** I believe it was a reasonable decision that gave her house a “belt and suspenders”- i.e. some redundancy, to help her and her family sleep better at night.

Hillside Retrofit Economics

Some hillside homes are almost beyond hope of an adequate seismic retrofit due to high landslide risk or a combination of structural problems. It is possible that effective strengthening measures could cost in the hundreds of thousands of dollars if the owner wants to really mitigate their slope stability or significant structural weaknesses. There is little benefit to retrofitting a home structurally if the ground it sits on is unstable.

The earthquake risk of hillside homes varies significantly from house to house and from site to site, and the cost of a necessary seismic retrofit can vary from $0 (no retrofit necessary) to extremely expensive. The decision to upgrade, move away, or live with the risk is a personal decision based on life-safety concerns, risk tolerance, and personal economics.

Due to the variability of cost and the many factors affecting the seismic risk of hillside homes, there is a need for good, up-front information for hillside homeowners.

4. Get More Information.

I hope the information in these blog posts about hillside homes is helpful for making decisions. Since the information is not house-specific, however, many need to go a step further.

Consulting a structural and/or geotechnical engineer is appropriate, and I am happy to do this. My preferred approach is to use a developed seismic risk assessment methodology. I currently use FEMA P-50, P-58, and ASCE 41, depending on the situation. You can learn more about these assessments here.

I believe seismic risk assessments have great value, and are a good first step in the decision process. If you hire me for an assessment, you will get a structural engineer’s opinion (a qualitative assessment) as well as an analytical (quantitative) assessment. This first-pass information can be done quickly at a relatively low cost, to help develop the “big picture” of what a potential retrofit would look like and what the potential benefits are.

“FEMA P-50” is a good seismic methodology that applies to most hillside homes. It will grade the house (with a letter grade from A to D-) based on how well it will perform in our largest expected earthquake. When I assess a home this way, I develop retrofit concepts and then grade the house pre-retrofit and post-retrofit. Sometimes I will provide a “lean” retrofit option, in addition to a more thorough retrofit option, if that makes sense for the particular structure.

Even if your hillside home was engineered relatively recently, it doesn’t hurt to have it double-checked. Engineers make mistakes sometimes, and hillside home retrofits can be difficult to design correctly.

If you’ve read all three of my blog posts on hillside homes, you can hopefully tell that I’m trying to sound the alarm regarding earthquake risk with these types of homes. However, not all hillside homes are in danger.

My main point is that there are many variables to seismic risk with these unique structures. To make an informed decision about what to do, hillside homeowners need accurate information that takes all these variables into account. This information may lead you in many different directions depending on your specific house and personal situation.